This invention relates to a piezoelectric device, and particularly relates to a piezoelectric device vibrating in the thickness mode of vibration at a pre-selected frequency with suppressed unwanted vibration responses.
The resonance frequency of a piezoelectric plate vibrating in the thickness mode is inversely proportional to the thickness of the plate. Accordingly, by using a thin piezoelectric plate, the piezoelectric device can be widely used in a high frequency range.
However, conventionally such a thickness mode piezoelectric device has a defect in that it is difficult to suppress or to eliminate the unwanted vibration responses which exist in the wanted thickness vibration responses at/or near that response.
Several methods have been proposed to overcome this defect. For example, U.S. Pat. No. 2,249,933 discloses a method of making the electrode area smaller than one half of the area of the piezoelectric plate. U.S. Pat. No. 3,384,768 discloses a method of making the resonance frequency of a region of resonator where the electrode is applied lower than that of the region having no electrodes thereon. On the other hand, U.S. Pat. No. 3,363,119 discloses a method of providing a recess and electrodes on the piezoelectric plate so as to make the resonance frequency range of the portion of the plate having the electrode thereon lower than the range of the portion of the plate having no electrode thereon. Further, the literature entitled "High Frequency Crystal Filters Employing Multiple Mode Resonators Vibrating in Trapped Energy Mode," Proc. 20th Annual Symposium on Frequency Control, pp 266 to 287, April 1966 discloses a piezoelectric device having a symmetric and anti-symmetric trapped mode, which has a structure with a plurality of partially limited electrodes on a piezoelectric plate.
Although many efforts to overcome the problem have been made, it is still difficult to eliminate successfully the unwanted vibration responses from any piezoelectric plate. Especially it is difficult to eliminate successfully the unwanted vibration responses from the harmonic overtones of the thickness mode of any piezoelectric plate.